How to Measure Ripeness

RIPENESS is usually referred to as the quality of being ready to be collected or eaten. Fruit/vegetable quality is a concept based on several criteria which depend on the objective. When do you harvest fruit/vegetables from a plant, to arrive into the supermarket in a ‘fresh picked’ condition? Every day, consumers perform a simple subjective test in an attempt to measure the quality of fruit/vegetables they wish to purchase by pressing the surface with the thumb or by squeezing the fruit/vegetable in the hand, thus utilising the principle of deformation in either a penetration or compression manner.

An alternative to this consumer assessment is the measurement of ‘freshness’ which gives an indication of age/softness which can be perceived as a negative quality. The flexing of a carrot or celery stick will allow the consumer to assess whether the fruit/vegetable bends, the degree of flexibility thereby indicating whether a product is flaccid and losing its desired textural quality.

Firmness is an important factor to take into account since most, if not all, fruits/vegetables exhibit a substantial change in firmness during the process of ripening. This change may considerably influence the consumer acceptance of the product as it is related to the ‘eating maturity’ and the fruit texture. Generally, if it is firm the fruit will be purchased, and if it is soft then the question will be, when to consume it by, or whether to purchase it at all.

From the producer’s point of view, firmness can be an indication of the shelf life of the product. Also of major consideration is the mechanical harvesting of fruits which can cause damage from branches and other fruits as fruit falls from the tree and drops on the ground. These damages are in the form of splits, punctures and bruises. Further damage is caused when it is raked, picked up, loaded and transported to distant places by trucks. Generally, it takes several days in transportation from one place to another that causes various changes in physico-mechanical properties of fruits. The post-harvest mechanical properties data of fruits and vegetables are important in adoption and design of various handling, packaging, storage and transportation systems.

For the fruit/vegetable processor, being able to measure different stages in the ripening process objectively and compare the texture of fruits from different sources can be of vital importance. Such information gives the processor more control over the supplier and allows the monitoring of specific textural characteristics.

Bioyield Point is a measurement that growers often use which indicates the initial cell rupture in whole fruit and may be used as a criterion for maximum allowable load that the sample can sustain without showing any visible damage. The rupture point indicates failure, the force and distance at this point indicating the load and deformation that can be applied before failure occurs. The Apparent modulus of elasticity of deformation is referred to as the crop stiffness or rigidity and is characterised by the tangent to the initial, more or less linear section of the curve. The energy (rupture energy) up to this point is the work required to cause rupture which indicates sample toughness. The bioyield energy is the resilience (deformation work) is a measure of the ability of the sample to store energy in the range of elasticity and is given as the area under the curve up to the bioyield point. The bioyield strength and strain are the stress and strain respectively measured at the bioyield point. Continued penetration into the sample provides a measurement of flesh firmness. The fruit energy concept has been used to design and specify cushioning materials for padding surfaces used for handling and transportation, while the modulus of deformation is associated with fruit stiffness and firmness.

Texture Analysis methods give food companies the ability to accurately assess the texture of fruits and vegetables during maturation, ripening and storage to identify whether a fruit or vegetable is capable of withstanding the stresses and strains of packaging, transportation and processing. At each stage a method for measuring the firmness of the skin, flesh, and core of the fruit/vegetable is necessary to ensure the maximum return from a crop. This can be done by a number of different methods depending upon the property that needs to be measured and the form that the sample takes.

Typical Probe/Fixture used for Measurement of Ripeness

The test type is chosen according to the size, form, homogeneity etc. of the sample and can therefore be derived from a single piece compression, bulk compression, bending, penetration or cutting tests as is most appropriate. Whilst performing one of these tests it is not uncommon to be able to obtain a number of other textural parameters that are also possessed by the product.

For example:

Flexibility would be assessed by imitating what the consumer does using a Three Point Bend Rig, like this:

The assessment of fruit/vegetable firmness could be measured by a single or multiple penetration test depending on the homogeneity of the sample. Bioyield point (force to puncture the skin) is usually performed with a single penetration test and has traditionally involved penetration probes called ‘Magness-Taylor’ probes.

Sometimes it is advantageous to test a number of samples at a time to optimise repeatability of measurement:

Other measurements of interest might be ‘Detachment Force’ – the force to remove produce in order to harvest or the force to bite into a sample (‘Bite Force’ using a Cutting Test), both of which can be imitated on a Texture Analyser easily.